Active radar systems, in particular for use as active imaging systems, are becoming increasingly popular at ultrasonic, microwave, millimeter and terahertz frequencies for a number of applications including medical and security applications. Security active imaging systems for example enable suspicious items hidden under clothes or in bags to be visualised and to be easily identified. Medical active imaging systems on the other hand enable the visualisation of a huge variety of biological items.
There are various active radar systems and methods known using various means for separating the different transmitted signals, in particular for a MIMO (Multiple Input Multiple Output) Radar or MIMO Imaging system.
J. H. G. Ender, J. Klare, “System Architectures and Algorithms for Radar Imaging by MIMO-SAR”, IEEE Radar Conference 2009 proposes two different methods. The first method time multiplexes the different transmitted signals to the different antennas, i.e. only one transmit antenna transmits a signal at one time. The second method transmits on all the transmitters at the same time, but each one transmits on a different band (which is a fraction of the overall system bandwidth). The mapping of bands to transmitter antennas may change from one time slot (or pulse duration) to the next.
J. Klare, O Saalmann, H. Wilden, “First Experimental Results with the imaging MIMO Radar MIRA-CLE X”, EUSAR Conference 2010 proposes to multiplex the different transmitted signals to the different antennas, i.e. only one transmit antenna transmits a signal at one time.
J. Klare, “Digital Beamforming for a 3-D MIMO SAR—Improvements Through Frequency and Waveform Diversity”, IEEE Geoscience and Remote Sensing Symposium (IGARSS 2008) proposes two different methods. The first method transmits on all of the transmitters at the same time, but each one uses a different band. The mapping of bands to transmit antennas changes from one time slot (pulse duration) to the next. The second method transmits on all of the transmit antennas at the same time but with Doppler tolerant orthogonal coded waveforms.
B. J. Donnet, I. D. Lonstraff, “MIMO Radar—Waveforms and applications”. 4th EMRS DTC Technical Conference—Edinburgh 2007 describes a MIMO Radar system using OFDM (Orthogonal Frequency Division Multiplex) which uses Doppler tolerant Costas codes (and Golay codes) for determining the frequency hopping patterns of OFDM different transmitted waveforms.
B. J. Donnert et al, “MIMO Radar, Techniques and Opportunities”, 3rd European Radar Conference proposes a MIMO system which uses OFDM and changes the frequencies on the different transmitted antennas in discrete steps. Different transmitters always send on different discrete frequencies.
J. H. Kim et al, “Investigation of MIMO SAR for Interferometry”, Proceedings of 4th European Radar Conference proposes a MIMO system in which the different transmit signals are separated by using a space time block code (STBC).
G. Brooker, “Understanding Millimeter Wave FMCW Radars”, 1st International Conference on Sensing Technology, Nov. 21-23, 2005, Palmerston North, New Zealand explains frequency modulated continuous wave (FMCW) radar systems. Such radars operate using the homodyne principle, i.e. a CW radar in which an oscillator serves as both the transmitter and local oscillator.
The above mentioned known systems have the drawbacks that they require much time since all transmit signal are to be transmitted subsequently one by one, require the use of special (complex, possibly non-optimal) codes, do no use the available bandwidth efficiently, are complex to implement, and/or require special (complex, expensive) hardware.